Most vehicle steering racks are manufactured from round solid bar stock, with the toothed region broached across the bar near one end. This results in the cross section of the toothed region having a ‘D’ shape and hence these racks are commonly referred to as “D-racks”. The toothed region of such a broached D-rack has significantly less bending strength than the round solid shank extending from it. However, to minimize the weight of the steering rack, it is desirable that the toothed region and the shank have similar bending strength. A common approach to this problem is to gun drill the shank over most of its length resulting in a substantially tubular shank. However, the disadvantages of gun drilling are that material is wasted and it is a relatively expensive process.
An alternative method of manufacturing a steering rack from round solid bar stock is to forge the toothed region. U.S. Pat. No. 4,571,982 (Bishop) and U.S. Pat. No. 5,862,701 (Bishop et al) disclose die apparatus for flashless warm forging the toothed region to net shape. “Net shape” means that the forged rack teeth do not require any further machining after forging. An advantage of forging is that the rack teeth may be shaped to have a variable gear ratio. The cross section of the toothed region of racks forged by this type of die has a ‘Y’ shape and such racks are commonly referred to as “Y-racks”. The toothed region of a forged Y-rack has greater bending strength than the toothed region of a D-rack broached from the same diameter solid bar, and so Y-racks can be forged from smaller diameter bar whilst maintaining overall bending strength. However, the shanks of Y-racks are still commonly gun drilled to further reduce weight.
Numerous attempts have been made to further reduce weight by the manufacture of hollow steering racks from tube stock. One such method used in limited production is disclosed in U.S. Pat. No. 4,598,451 (Takanosuke) where a series of mandrels is passed through a flattened tube to progressively fill an external tooth die. This method is expensive and time consuming and as such is not suited to high volume production. Furthermore, the size of teeth that can be produced is limited by the wall thickness of the tube and the method is not suited to producing racks with variable ratio teeth.
A “composite rack” is defined as a rack made by joining two or more members to each other. Typically a composite rack is made by joining a shank made from tube to a short solid rack member. Such composite racks have the advantages of reduced weight without the limitations of forming the rack teeth onto a tube. Various methods of making composite racks have been proposed or used in limited production. For example, a composite steering rack has been used in Honda “Odyssey” vehicles. This rack is made by welding a tubular shank to a short solid forged D-rack. Typically, steering racks for hydraulic power steering gears have an external circumferential groove approximately mid way along the shank for locating a hydraulic piston. A disadvantage of the Honda “Odyssey” rack is that since the tubular shank has a constant wall thickness along its length, the wall must be thick enough to machine this groove on its outside diameter without weakening the rack. The wall of the tube must also be thick enough to machine an internal thread at one end to attach a tie rod end. These localized limitations on wall thickness mean that a composite rack having a tubular shank of constant wall thickness cannot fully exploit the weight saving potential of a composite rack.
The present invention seeks to ameliorate at least some of the disadvantages of the prior art.